Fix visual accessibility issues 6 and 20 (#5725)

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## Why are these changes needed?

Fixes visual accessibility issues `(6) Luminosity contrast ratio of
focus indicator on 'Venv' control is less than 3:1.` and `(20) "Only
color is used to indicate the focus indicator on the 'Venv', and 'conda'
Tab controls."`

## Related issue number

#5633 

before:
<img width="162" alt="image"
src="https://github.com/user-attachments/assets/ac317e82-c82e-4d3b-910b-e9eb5d087340"
/>

![image](https://github.com/user-attachments/assets/2971043a-8b5b-4521-8135-c411c0ecab1f)

after:
<img width="160" alt="image"
src="https://github.com/user-attachments/assets/8053ae77-4d62-491b-9d78-657646ed71a6"
/>

![image](https://github.com/user-attachments/assets/5c3ac2c3-4813-461b-8958-4665860574a0)

python/packages/autogen-core/docs/src/_static/custom.css

@@ -66,13 +66,16 @@ html[data-theme="light"] .bd-header {
 .bd-content .sd-tab-set>input:checked+label {
   border: none;
   transform: translateY(0);
-  font-weight: 600;
-  border-bottom: 2px solid var(--pst-color-secondary);
-
+  font-weight: 700;
+  border-bottom: 4px solid var(--pst-color-secondary);
+}
+.bd-content .sd-tab-set>input:focus-visible+label {
+  border: 2px outset var(--pst-color-secondary);
+  transform: translateY(0);
 }
 .bd-content .sd-tab-set>label {
   background-color: transparent;
-  border: none;
+  font-weight: 500;
 }
 
 .card-title {